Single-beam gradient force traps have been demonstrated for neutral atoms and dielectric particles. Generally, the single-beam gradient force trap consists only of a strongly focused laser beam having an approximately Gaussian transverse intensity profile. In these traps, radiation pressure scattering and gradient force components are combined to give a point of stable equilibrium located close to the focus of the laser beam. Scattering force is proportional to optical intensity and acts in the direction of the incident laser light. Gradient force is proportional to the optical intensity and points in the direction of the intensity gradient.
Particles in a single-beam gradient force trap are confined transverse to the laser beam axis by a radial component of the gradient force. Stabilizing the particle along the axis direction of the trap is achieved by strongly focusing the laser beam to have the axial component of gradient force dominate the scattering force in the trap region.
In prior work using single-beam gradient force optical traps on dielectric particles, trapping was demonstrated with a visible light laser source (.lambda.=514.5 nm.) focused by a high numerical aperture lens objective. See A. Ashkin et al., Optics Letters, Vol. 11, p 288-90. The dielectric particles were closely spherical or spheroidal in shape and ranged in size from 10 .mu.m diameter Mie glass spheres (.alpha.&gt;&gt;.lambda.) down to 260 Angstrom diameter Rayleigh particles (.alpha..ltoreq..ltoreq..lambda.). Use of such regularly shaped particles in the Mie regime was desirable as taught in this and other articles.
For Mie particles, both the magnitude and direction of the forces depend on the particle shape. This restricts trapping to fairly simple shapes such as spheres, ellipsoids, or particles whose optical scattering varies slowly with orientation in the trap. In the Rayleigh regime, the particle acts as a dipole and the direction of force is independent of particle shape; only the magnitude of force varies with particle orientation.
It is not an insignificant result of the prior work that silica and other dielectric particles experienced varying amounts of irreversible optical damage from the trap. While it was suggested that the single-beam trap and the prior results would be extensible to biological particles, the resulting damage from exposure in the trap would destroy or significantly incapacitate the biological particles and render them useless. Also, since prior optical traps have been defined for quite regular-shaped, dielectric particles, their extension to biological particles is cast in doubt because regularity of shape is not an attribute of biological particles.